test(nextjs): Update type cast for integration test

[AbhiPrasad]

Currently on master tests are failing with:

./pages/users/[id].tsx:55:31
Type error: Object is of type 'unknown'.

  53 |     return { props: { item } };
  54 |   } catch (err) {
> 55 |     return { props: { errors: err.message } };
     |                               ^
  56 |   }
  57 | };
  58 |

To fix this, we cast err to Error type.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.57 KB (+0.01% 🔺)
@sentry/browser - Webpack	22.57 KB (0%)
@sentry/react - Webpack	22.6 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	29 KB (+0.01% 🔺)